The present invention relates to a photothermographic material. In particular, it relates to a photothermographic material that can provides sufficient image density and can sufficiently suppress coloration of blank portions during storage in the dark after development.
In recent years, reduction of amount of waste processing solutions is strongly desired in the medical diagnosis field and the photographic art field from the standpoints of environmental protection and space savings. Techniques relating to photosensitive thermographic materials for use in medical diagnosis and photographic-art processes are required which enables efficient exposure by a laser image setter or a laser imager and formation of a clear black image having high resolution and sharpness. The photosensitive thermographic materials can provide users with a simple and non-polluting heat development processing system that eliminates the use of solution-type processing chemicals.
The same is demanded in the field of ordinary image-forming materials. However, photo-images for medical use require high image quality excellent in sharpness and graininess as they need very fine images. In addition, for easy diagnosis, cold monochromatic images are preferred. At present, various types of hard copy systems using pigment and dye, for example, ink jet printers and electrophotographic systems are available as ordinary imaging systems. However, no satisfactory image-forming system is available for medical use.
Meanwhile, methods utilizing a silver salt of an organic acid for forming an image by heat development are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Klostervoer, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d, Imaging Processes and Materials, Neblette, 8th ed., compiled by J. Sturge, V. Walworth and A. Shepp, Chapter 9, p.279, (1989). The photothermographic material, in particular, comprises a image-forming layer (photosensitive layer) containing a photocatalyst (e.g., silver halide) in a catalytically active amount, a reducing agent, a reducible silver salt (e.g., silver salt of an organic acid), and optionally a toning agent for controlling tone of silver, which are usually dispersed in a binder matrix. When the photothermographic material is heated at a high temperature (e.g., 80xc2x0 C. or higher) after imagewise light exposure, a monochromatic black silver image is produced through an oxidation-reduction reaction between the silver halide or the reducible silver salt (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image of silver halide generated upon exposure. Therefore, the monochromatic silver images are formed in exposed areas of the materials. This technique is disclosed in many references including U.S. Pat. No. 2,910,377 and Japanese Patent Publication (Kokoku, hereinafter referred to as JP-B) 43-4924, and Fuji Medical Dry Imager FM-DP L was put into the market as an image-forming system for medical diagnosis utilizing photothermographic materials.
Because the aforementioned photothermographic materials are not subjected to fixation after heat development, the thermally reactive organic acid silver salt and reducing agent are left in the photothermographic materials as they are. Thus, they suffer a problem that blank portions thereof are colored if the materials are stored for a long period of time after the heat development. Phenol type reducing agents (see, for example, European Patent Publication EP0803764A1, Japanese Patent Laid-open Publication (Kokai, hereinafter referred to as JP-A) 51-51933, JP-A-6-3793 etc.) are effectively used for the photothermographic materials because of their high reactivity, and the coloration of blank portions can effectively be suppressed by reducing the amount thereof to be used. However, if the amount of o-bisphenol type reducing agent to be used is reduced, it becomes impossible to obtain sufficient image density, and thus image storability is difficult to be compatible with image density.
Therefore, an object of the present invention is to provide a photothermographic material that can provide sufficient image density at practical reaction temperatures (specifically 100-140xc2x0 C.) with practical reaction times (specifically 1-30 seconds), and can sufficiently suppress coloration of blank portions during storage in the dark after development.
The inventors of the present invention conducted extensive studies to achieve the aforementioned object. As a result, they found that, if a phenol compound, which is used as a reducing agent, and a compound having a particular hydrogen bond formation rate constant were used in combination in photothermographic materials, sufficient image density could be obtained and image storability could be markedly improved without substantially degrading the reducing property. Thus, they accomplished the present invention.
That is, the present invention provides a photothermographic material comprising, on one side of a support, a photosensitive silver halide, a non-photosensitive silver salt of an organic acid, a reducing agent for silver ions and a binder, wherein it contains one or more phenol compounds as the reducing agent and one or more compounds satisfying at least one of the following requirements A and B in combination:
A: the hydrogen bond formation rate constant Kf is 20-4000,
B: the chemical structure is represented by the following formula (II), (III), (IV) or (V), or has a phosphoryl group: 
In the formula (II), R21 and R22 independently represent an alkyl group, and R23 represents an alkyl group, an aryl group or a heterocyclic group. Two or more of R21, R22 and R23 may be taken together to form a ring.
In the formula (III), R31 and R32 independently represent an alkyl group, an aryl group or a heterocyclic group. R31 and R32 may be taken together to form a ring.
In the formula (IV), R41 and R42 independently represent an alkyl group, an aryl group or a heterocyclic group. R43 represents an alkyl group, an aryl group, a heterocyclic group or xe2x80x94N(R44) (R45). R44 and R45 independently represent an alkyl group, an aryl group or a heterocyclic group. Two or more of R41, R42, R43, R44 and R45 may be taken together to form a ring.
In the formula (V), R51, R52, R53, R54 and R55 independently represent a hydrogen atom or a substituent. Two or more of R51, R52, R53, R54 and R55 may be taken together to form a ring.
The phenol compound contained in the photothermographic material of the present invention is preferably an o-polyphenol compound, in particular, a compound represented by the following formula (I). 
In the formula, R1, R2, R3, R4, R5, R6, R7 and R8 independently represent a hydrogen atom or a group that can substitute on a benzene ring, and L represents a group xe2x80x94Sxe2x80x94 or a group xe2x80x94CHR9xe2x80x94 where R9 represents a hydrogen atom or an alkyl group.
As the compound represented by the formula (I), preferred are those compounds where R2, R4, R5 and R7 are hydrogen atoms, R1 and R8 independently represents an alkyl group, R3 and R6 independently represents an alkyl group, and L is xe2x80x94CHR9xe2x80x94. Particularly preferred are those compounds where R1 and R8 independently represent a secondary or tertiary alkyl group.
The photothermographic material of the present invention preferably contains a compound of which hydrogen bond formation rate constant Kf is 70 to 4000.
Further, the photothermographic material of the present invention preferably contains an o-polyphenol compound and one or more compounds having a phosphoryl. The compound having a phosphoryl group is preferably a compound represented by the following formula (VI): 
In the formula, R61, R62 and R63 independently represent an alkyl group, an aryl group, an aralkyl group, an alkoxy group, an aryloxy group, an amino group or a heterocyclic group.
In the present specification, xe2x80x9c-xe2x80x9d indicates a range including numerical values mentioned before and after it as the minimum and maximum values, respectively.
By using a phenolic compound and a compound having a hydrogen bond formation rate constant Kf of 20-4000 or represented by the formula (II), (III), (IV) or (V) or a compound having a phosphoryl group in combination, it became possible to provide a photothermographic material that can provide sufficient image density at a practical reaction temperature and within a practical reaction time, and can sufficiently suppress coloration of blank portions after development and storage in the dark.